This chapter guides the reader to the material in this book.  It begins by outlining the meaning and rationale for agent-based models/modelling (ABM), focusing on their history, how they evolved and how they sit within the broader context of modelling and simulation for geographical systems. Three themes which we see essential to ABM are then outlined, namely the question of detail versus model and data parsimony of which ABM represents the former, questions of model validation that flow from this, and lastly issues about the extent to which ABM is a generic or specific style in terms of applications. We examine the essence of such models in terms of the way behaviour is modelled using various rules, and then we discuss technical issues such as computation, visualization, error, and schemes for model design. All this sets the context for the various chapters that follow. We conclude by explaining briefly what is contained in each chapter and by guiding the reader in how best to use this book.